Apparatus and method for modifying a power flow in a section of an electric power line

ABSTRACT

The invention concerns an apparatus and a method for modifying a power flux in an electrical transmission line section ( 6,8 ), each section including phase lines having n conductors ( 10,12 ) electrically insulated from one another and shorted at the ends ( 40 ) of the section. The apparatus comprises a power exchange unit including a power converter ( 18 ) for power conversion between first and second pairs of terminals ( 20,22 ), the first pair of terminals ( 20 ) being connected in series to at least one conductor of the section; and an electrical element ( 24 ) connected to the second pair of terminals ( 22 ) and adapted to circulate power through the power converter to modify said power flux.

The present invention relates to an apparatus and to a method formodifying a power flow in a segment of an electric power line withmultiple phase conductors. In the present text, we will refer to “phaseline” to describe what is commonly known by a person skilled in the artas “phase”. The apparatus and method are used for modifying the powerflow in a segment of an electric power line and also, but notexclusively, de-ice an electric power line, to modify the power flowthrough an electric power line in a static or dynamic manner, tostabilize an electric power network, to filter harmonics of an electricpower line, to absorb or dissipate power transmitted by an electricpower line, or even to limit the electric current of an electric powerline.

Known in the art, there is U.S. Pat. No. 6,486,569 B2 granted on Nov.26, 2002, and naming Mr. Pierre COUTURE as inventor, who is also theinventor of the present invention. This patent describes a method ofmanaging a power flow in an electric power network. The method comprisesthe steps of providing switching units that are mounted on insulatedportions of pylons of electric power lines for switching the current ofat least one conductor of a plurality of phase conductors that areelectrically insulated from one another, said switching of conductorsenabling a change of impedance which modulates the power flow. Themethod also comprises a step of managing the power flow in the segmentsof the electric power line, by changing the series impedance of the lineby controlling switching units.

Also known in the art, there is international publication No. WO02/41459 A1, published on May 23, 2002, and naming Mr. Pierre COUTURE asinventor, who is also the inventor of the present application. Thispatent describes a switching apparatus and method for varying theimpedance of a phase line of a segment of an electric power line. Foreach of at least one of the n conductors of the phase line, a passivecomponent and a pair of electromechanical and electronic switches areprovided. The pair of switches is adapted to selectively connect anddisconnect the passive component in series with the correspondingconductor, in response to control signals. Means are also provided forcontrolling each pair of switches according to current operationconditions of the phase line.

Also known in the art, there is international publication No. WO00/35061, published on Jun. 15, 2000, and naming Mr. Pierre COUTURE asinventor, who is also the inventor of the present application. A U.S.patent corresponding to this international application has also beengranted on May 28, 2002 under No. U.S. Pat. No. 6,396,172 B1. Thesedocuments describe a switching apparatus and method for a segment of anelectric power line comprising several phase lines. Each phase linecomprising several conductors electrically insulated from one anotherand connected in parallel for conducting a phase current. Pairs ofelectromechanical and electronic switches connected in parallel areprovided for selectively opening and closing the conductors of eachphase line so as to conduct the corresponding phase current through oneor several conductors. Control means are also provided for controllingthe pairs of electromechanical and electronic switches according tocurrent operating conditions of the segment.

One of the drawbacks that can be found in most of the above-mentionedapparatuses and methods, resides in the fact that the strategiesavailable to the user for modifying the power flow in a segment of anelectric power line are limited. Furthermore, the operating speed ofthese apparatuses is limited according to the frequency of the currentcirculating in the power line.

One of the objects of the present invention is to propose an apparatusand a method for modifying the power flow in a segment of an electricpower line according to a range of possibilities much greater than whatis possible in the prior art, in an efficient and safe manner.

Another object of the present invention is to quickly modify the powerflow than what is possible in the prior art. Thus, for a power lineoperating at 60 hertz, the modification of the power flow can be done ina period of time shorter than 8 milliseconds without waiting for thepassage of the current by zero.

Another object of the present invention, which is obtained by apreferred embodiment, is to propose an apparatus and a method formodifying a power flow in a segment of an electric power line, so that apower flow of said segment is sent directly towards another segment ofthe electric power line.

These objects, advantages, and other features of the invention willbecome more apparent upon the reading of the following non restrictivedescription, of different preferred embodiments given as examples only,with reference to the attached drawings.

SUMMARY OF THE INVENTION

According to the present invention, there is provided an apparatus formodifying a power flow in a segment of an electric power line, eachsegment including phase lines each having n conductors electricallyinsulated from one another and short-circuited at ends of the segment,the apparatus comprising a power exchange unit including: a powerconverter for converting power between first and second pairs ofterminals, the first pair of terminals being connected in series with aconductor of the segment; and an electric component connected to thesecond pair of terminals and capable of circulating power through thepower converter for modifying said power flow.

According to the present invention, there is also provided a method formodifying a power flow in a segment of an electric power line, each thesegment having phase lines each having n conductors insulated from oneanother and short-circuited at ends of the segment, the methodcomprising the following steps:

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a schematic circuit diagram showing two segments of anelectric power line, and an apparatus according to a preferredembodiment of the present invention.

FIG. 2 is a schematic circuit diagram showing two segments of anelectric power line, and an apparatus according to a preferredembodiment of the present invention.

FIG. 3 is a schematic circuit diagram showing two segments of anelectric power line, and an apparatus according to a preferredembodiment of the present invention.

FIG. 4 is a schematic circuit diagram showing two segments of anelectric power line, and an apparatus according to a preferredembodiment of the present invention.

FIG. 5 is a schematic circuit diagram showing two segments of anelectric power line, and an apparatus according to a preferredembodiment of the present invention.

FIG. 6 is a schematic circuit diagram showing two segments of anelectric power line, and an apparatus according to a preferredembodiment of the present invention.

FIG. 7 is a schematic circuit diagram showing two segments of anelectric power line, and an apparatus according to a preferredembodiment of the present invention.

FIG. 8 is a schematic circuit diagram showing two segments of anelectric power line, and an apparatus according to a preferredembodiment of the present invention.

FIG. 9 is a schematic circuit diagram showing two segments of anelectric power line, and an apparatus according to a preferredembodiment of the present invention.

FIG. 10 is a side view of a pylon supporting an electric power line ontowhich an apparatus according to a preferred embodiment of the presentinvention is mounted.

FIG. 11 is a perspective view of a component of the apparatus shown inFIG. 10.

FIG. 12 is a perspective view of another component of the apparatusshown in FIG. 10.

FIG. 13 is a front view of the pylon shown in FIG. 10.

DETAILED DESCRIPTION OF THE DRAWINGS

Referring now to FIG. 1, there is shown a schematic circuit diagramshowing two segments 6 and 8 of an electric power line. An electricpower line can of course be segmented into a multitude of segments.Generally, an electric power line comprises three phase lines A, B andC. In this FIG. 1 and in FIGS. 2 to 9, in order to avoid overloading thefigures, a single phase line A is shown by segment even if a segmentnormally comprises several phase lines. Furthermore, it is important towell understand that for each case shown in FIGS. 1 to 9, an apparatusaccording to the present invention can be installed on each phase lineof the segment. Each phase line has n conductors 10 and 12 electricallyinsulated from one another and short-circuited at ends 14 of eachsegment. For each segment, a circuit equivalent to the self-inductanceL, the mutual inductance M and the resistance R is indicated. For thesake of discussion, the capacitive effects of the line are neglected.The phase line A of the segment 6 is provided with an apparatusaccording to a preferred embodiment of the present invention.

The apparatus according to the present invention comprises a powerexchange unit 16 having a power converter 18 for converting powerbetween first and second pairs of terminals 20 and 22. The pair ofterminals 22 is connected in series with at least one conductor 12 ofthe segment 6. The power exchange unit also includes an electriccomponent 24 connected to the pair of terminals 22. This electriccomponent 24 is capable of circulating power through the power converter18 for modifying the power flow through the segment 6 of the electricpower line. Preferably, the power exchange unit further comprises aswitch 26 for selectively connecting and disconnecting the pair ofterminals 20 in series with the conductor 12 of the segment 6, inresponse to control signals. Preferably, the apparatus comprises n−1power units, n being the number of conductors forming a segment. Forexample, in the present case, the phase line A of the segment 6 isformed by two conductors hence the number n is equal to 2.

In general, according to this preferred embodiment, the n−1 power unitsare connectable by means of their switch 26 respectively to n−1conductors of a phase line belonging to the segment. In the presentcase, the number of conductors being equal to 2, the power exchange unitis capable of being connected by means of its switch 26 to the conductor12 of the phase line A being part of the segment 6. Preferably, theelectric component 24 is selected among the following electriccomponents: a capacitor 28, a battery 30, an inductance 32, a resistance34 and a resistance connected in parallel to a capacitor 36. Preferably,the converter 18 is a converter integrated to a phase line of thehigh-voltage power line.

According to the present invention, preferably, each segment is composedof a transmission line having several phase lines onto which anapparatus according to the present invention is mounted. The line isused as a work head transformer. The converter 18 along with theelectric component 24 act as an energy source or drain controllable involtage or in current. The converter controls the phase, the amplitudeand in certain cases the frequency so as to produce functions knownunder the English appellation “Flexible AC Transmission System” (FACTS).A FACTS carries out a control of the impedance, a stabilization, afiltration, a limitation of current, a braking, etc. The electriccomponent may also fulfill the function of a storage unit when it iscarried out for example by a capacitor, a battery, a fuel cell, etc. Thereplacement of a storage unit by an electric resistance or by acombination of a resistance and a capacitor, which extract active power.The electric resistance can be a variable resistance.

The converter 18 can consist of a simple diode bridge, thyristors or amodulator by variable width pulses (PWM). The switches (not shown) usedin the converter can be diodes, thyristors, Insulated Gate BipolarTransistors (IGBT), Metal-Oxide Semiconductor Field Effect Transistors(MOSFET), Metal-Oxide Semiconductor Controlled Thyristors (MCT), GateTurn Off Semiconductors (GTO), etc. The converter 18 as well as theelectric component 24 are mounted directly on the phase line A of thesegment 6 without reference to the ground or to the other phases. Theelectric component 24 when acting as a storage unit can be a capacitor,an inductance, a battery or a fuel cell. The use of a segment of anelectric power line as a coupling transformer and the use of anapparatus according to the present invention lowers the carrying outcosts of a FACTS. The segments with converters can be distributed alonga transmission line. This technology offers the advantage of acting onlyon a portion of the transmitted power on the line. This fraction ofaffected power depends on the number of segments having converters andon the number of conductors with converters.

The voltage at the terminals of the converters is proportional to thelength of the segment and to the current in the phase line. Thedistribution of the segments with converters along an electric powerline reduces the costs of the converters in addition to allowing aserial production of the converters. This distribution of the convertersalong an electric power line increases the reliability of the systembecause the loss of a converter does not affect the operation of theother converters located along the line and the other converterscontinue to modify the power flow through the line with a reducedmodulation envelope. Given the fact that the power units according tothe present invention can be installed directly on the line, this allowsa reduction of the space used on the ground. Preferably, for a segmenthaving three phase lines, an apparatus according to the presentinvention acts simultaneously on the three phases. However, in certaincases, the apparatus according to the present invention could act onlyon one or two of the phases, for balancing one or several phases.

The switch 26 that can be a mechanical, electronic or electromechanicalswitch, can be used for re-establishing the line into its original statewhen required. In the case of a segment with more than two conductorsper phase line, each of the conductors can be linked to a converter orthey can be regrouped according to different configurations. Forexample, for a 735 kV line with four conductors per phase line, one canprovide different layouts of simple or double converters that will bepresented in reference to FIGS. 2 to 9.

The present invention also provides a method for modifying power flow ina segment of an electric power line. The method comprises step a) ofproviding a power unit that includes a power converter 18 for convertingpower between the first and second pairs of terminals 20 and 22, and anelectric component 24 connected to the second pair of terminals 22 andcapable of circulating power through the power converter. The first pairof terminals is of course connected in series with the conductor 12 ofthe phase line A of the segment 6. The method also comprises step b) ofconverting power between the first and second pairs of terminals 20 and22 by means of the power converter 18 for modifying said power flow inthe segment 2. Preferably, the method further comprises step c) ofselectively connecting and disconnecting the first pair of terminals 20in series with the conductor 12 by means of the switch 26, in responseto control signals. Preferably, in step a), n−1 power exchange units areprovided, in step b), the power is converted by at least one of the n−1power converters, and in step c), the n−1 power units are connected anddisconnected by means of their switch 26 respectively to n−1 conductorsof the phase line A of the segment 6.

Referring now to FIG. 2, the apparatus according to the presentinvention comprises at least one additional power unit for forming a setof n power units. In this case, the number n is equal to 2. The twopower units 15 and 16 are capable of being connected by means of theirswitch 26 respectively to the two conductors 10 and 12 of the phase lineA belonging to the segment 6.

In the present case, for safety reasons, so as to prevent that all theconductors of a same segment open simultaneously, it is important that asame switching device regroup the switches 26 associated to a same phaseline. This safety switching device which prevents the simultaneousopening of all the conductors of a same phase line could be for examplethe one proposed in international patent application published under No.WO 00/35061 corresponding to U.S. Pat. No. 6,396,172 B1.

In the present case, according to the method proposed by the presentinvention, in step a), an additional power unit is provided for forminga set of two power units 15 and 16; in step b), the power is convertedby at least one of the two power converters 18; and in step c), the twopower units 15 and 16 are connected and disconnected by means of theirswitch 26 respectively to the two conductors 10 and 12 of the phase lineA of the segment 6. It is worth noting that in the present embodiment,preferably of course, the converter 18 and the electric component 24 ofthe power unit 15 can be removed from the current flow path by closingthe switch 26. The closed switch creates short circuit across the powerexchange unit, and the conductor operates as though the power exchangeunit is not present. The layout shown in FIG. 2 could de-ice theconductors of the phase lines of the segment 6 and to carry out FACTSfunctions.

Referring now to FIG. 3, the apparatus according to the presentinvention comprises an additional power unit 15 for forming a set of twopower units 15 and 16. The two power units 15 and 16 are capable ofbeing connected by means of their switch 26 respectively to theconductor 12 of the phase line A belonging respectively to the segments6 and 8. The two power units 15 and 16 share a common electric component24 for allowing a power flow between the two segments 6 and 8.

In this FIG. 3, are also shown controllers 40, a supply 42, and atransceiver 44. The previously mentioned components with sensors (notshown) control the converters 18 from a control station (not shown) soas to carry out FACTS functions for controlling the power flow in thesegments 6 and 8, and as a result controlling the power flow in anelectric power network. The supply 42 operates by capacitive coupling orby inductive coupling or with the help of a solar panel or a combinationof these components. Of course, the controllers 40, the supply 42 andthe transceiver 44 can be used with each of the embodiments shown inFIGS. 1 to 9. These control components, whether the controller 40, thesupply 42 and the transceiver 44, is carried out by what is proposed inU.S. Pat. Nos. 6,396,172 B1 and 6,727,604 B2 (continuation in part).

The embodiment shown in FIG. 3 comprises a double converter integratedto a phase line of a transmission line operating at high and veryhigh-voltage. The active or reactive power extracted from the segment 6is stored in the electric component 24 and injected into the segment 8with the help of the converter 18 of the power unit 15. The phase, thevoltage or current amplitude and the frequency can be controlledindependently at the converter 18 of the unit 16 or at the converter 18of the unit 15 depending on the desired effects. Normally, the terminals20 are adapted to receive an alternating current and the terminals 22are adapted to receive a direct current. However, it is not excludedthat the terminals 22 may be adapted to receive an alternating current.The active or reactive power could also be inverted and thus travel fromthe segment 8 to the segment 6 through the converter 18 of the units 15and 16. In other cases, the two converters can operate in parallel andextract or inject the power of the electric component 24 that mayconstitute a storage or dissipative module. The two converters and theelectric component can be located in the same casings so as to reduceelectromagnetic emissions and costs. The system provides the electricnetwork with controlling functions of static or dynamic power flow, tocarry out harmonic or subharmonic filtering functions, to carry outdissipative functions, to allow the de-icing of the lines, etc. Theconverters communicate remotely or through an optical fibre.

The apparatus shown in FIG. 3 can be used for carrying out a nondissipative sub-synchronous filtering. The converter 18 of the unit 16then extracts from the segment 6 power of the subharmonic in such a wayas to reduce this subharmonic and to transfer it to the electriccomponent 24 which, in the current embodiment, is a capacitive storageunit. The converter 18 of the unit 15 then takes this power from theelectric component 24 and re-injects it at the frequency of the networkin the segment 8.

The apparatus shown in FIG. 3 can be installed on a bus in a junctionpost of several electric power lines. Thus, in the case where thesegments 6 and 8 belong to two distinct power lines, it is possible,with the apparatus shown in FIG. 3, to a transfer power from one of thetransmission lines to another.

In the present case, according to the method of the present invention,in step a), an additional power unit is provided for forming a set oftwo power units 18; in step b), the power is converted by means of twopower converters 18; and in step c), the two power units are connectedand disconnected by means of their switch 26 respectively to theconductor 12 of the phase line A belonging respectively to the segments6 and 8, the power units 15 and 16 sharing a common electric component24 for enabling a power flow between the two segments 6 and 8.

Referring now to FIG. 4, the switch 26 of each power unit 15 or 16 isable to connect and disconnect, for the phase line A of thecorresponding segment 6 or 8, n−1 conductors. In the present case, thenumber of conductors being equal to four, three conductors areshort-circuited among each other on each side of the correspondingswitch. In the present case, according to the method of the presentinvention, in step c), the switch 26 of each power unit 15 or 16connects and disconnects, for the phase line A of the segment 6 or 8,three conductors that are short-circuited among each other on each sideof the switch.

Referring now to FIG. 5, the switch 26 of each power unit 15 or 16 isadapted to connect and disconnect, for the phase line A of thecorresponding segment 6 or 8, more than one and at most n−1 conductorsthat are short-circuited among each other on each side of thecorresponding switch 26. In the present case, the number of conductorsis equal to four, and two of the four conductors are short-circuitedamong each other on each side of the corresponding switch. In thepresent case, according to the method of the present invention, in stepc), the switch 26 of each power unit 15 or 16 connects and disconnects,for the phase line A of the corresponding segment 6 or 8, two conductorswhich are short-circuited among each other on each side of the switch26.

Referring now to FIGS. 6 and 7, the switch 26 is adapted to connect anddisconnect, more than one and at most n−1 conductors of the phase line Aof the segment 6, said more than one and at most n−1 conductors beingshort-circuited among each other on each side of the switch 26. In thepresent case, in FIG. 6, there are four conductors, three of the fourconductors being short-circuited among each other on each side of theswitch 26. In the case of FIG. 7, there are four conductors, but onlytwo of the four conductors are short-circuited among each other on eachside of the switch 26. In the present case, according to the method ofthe present invention, in step c), the switch 26 connects anddisconnects three (FIG. 6) or two (FIG. 7) conductors of the phase lineA belonging to the segment 6. The three (FIG. 6) or the two (FIG. 7)conductors are short-circuited among each other on each side of theswitch 26.

Referring now to FIG. 8, the apparatus comprises at least one additionalpower unit for forming a first set of n−1 power units. In the presentcase, given the fact that the number of conductors is four, the firstset of power units is composed of three units. The switches 26 of thethree power units are adapted to connect and disconnect respectivelythree conductors 11, 12 and 13 of the phase line A belonging to thesegment 6. The three conductors 11, 12, and 13 are short-circuited amongeach other on one side of the switches 26. In the present case,according to the method of the present invention, in step a), twoadditional power units are provided to form the first set of three powerunits; in step b), the power is converted by the three power converters18; and in step c), the three power units 16 are connected anddisconnected by means of their switch 26 respectively to the threeconductors 11, 12, and 13 of the phase line A belonging to the segment6, the three conductors 11, 12, and 13 being short-circuited among eachother on one side of the switch 26.

Referring now to FIG. 9, the apparatus comprises at least one additionalpower unit for forming a second set of n−1 power units. In the presentcase, given the fact that the number n of conductors of the phase line Ais equal to four, three additional power units 15 are required forforming the second set of three power units 15. The switches 26 of thethree power units 15 of the second set are adapted to connect anddisconnect the three conductors 11, 12, and 13 of the phase line Abelonging to the segment 8. The three conductors of the phase line A ofthe segment 8 are short-circuited among each other on one side of theswitches 26 of the units 15. The first and second sets of power units 15and 16 share common electric components 24 to allow a power flow betweenthe two segments 6 and 8.

In the present case, according to the method of the present invention,in step a), three additional power units 15 are provided to form thesecond set of three power units 15; in step b), the power is convertedby at least two of the power converters 18 that belong respectively tothe first and second sets and that are linked by a common electriccomponent 24; in step c), the three power units 15 of the second set areconnected and disconnected by means of their switch 26 respectively tothe three conductors 11, 12 and 13 of the phase line A belonging to thesegment 8. The three conductors 11, 12 and 13 of the phase line A of thesegment 8 are short-circuited among each other on one side of thecorresponding switches 26. Said at least two power converters provides apower flow between the segments 6 and 8.

Referring now to FIGS. 10 to 13, there is shown a pylon provided with anapparatus according to the present invention. The converters 18 and theelectric component 24 can be mounted as shown in these figures. Theconverters 18 are supported by supports 52. In FIG. 10, there is shownthat the converters 18 do not need to support the mechanical tensionthat is present in the transmission line 94. Jumpers 50 support theconverters 18. The electric components 24, such as capacitors,resistances or batteries when required, can be located within the spacedefined by the conductors of a phase line of the segment as shown inFIGS. 10 and 11. If the power unit to be installed is relatively large,the use of a platform may be considered.

1. Apparatus for modifying a power flow in a segment of an electricpower line, each segment including phase lines each having n conductors,where n is equal to or greater than 2, electrically insulated from oneanother and short-circuited at ends of the segment, the apparatuscomprising a power exchange unit including: a power converter forconverting power between first and second pairs of terminals, the firstpair of terminals being connected in series with at least one of the nconductors of the segment; and an electric component connected to thesecond pair of terminals and capable of circulating power through thepower converter for modifying said power flow.
 2. Apparatus according toclaim 1, wherein the power exchange unit further comprises a switch forselectively connecting and disconnecting the first pair of terminals inseries with said at least one conductor of the segment, in response to acontrol signal.
 3. Apparatus according to claim 2, comprising n−1 powerexchange units connectable by means of their switch respectively to n−1of the n conductors of a phase line belonging to the segment. 4.Apparatus according to claim 2, comprising at least one additional powerexchange unit for forming a set of n power exchange units, the n powerexchange units being connectable by means of their switch respectivelyto n conductors of a phase line belonging to the segment.
 5. Apparatusaccording to claim 2, comprising an additional power exchange unit forforming a set of two power exchange units, the two power exchange unitsbeing connectable by means of their switch respectively to conductors oftwo phase lines belonging respectively to said segment and to anothersegment, the two power exchange units sharing a common electriccomponent for allowing a power flow between the two segments. 6.Apparatus according to claim 5, wherein the switch of each powerexchange unit is able to connect and disconnect, for the correspondingphase line, n−1 conductors that are short-circuited among each other oneach side of the switch.
 7. Apparatus according to claim 5, wherein theswitch of each power exchange unit is able to connect and disconnect,for the corresponding phase line, more than one and at most n−1conductors that are short-circuited among each other on each side of theswitch.
 8. Apparatus according to claim 2, wherein the switch is able toconnect and disconnect more than one and at most n−1 conductors of aphase line belonging to the segment, said more than one and at most n−1conductors being short-circuited among each other on each side of theswitch.
 9. Apparatus according to claim 2, comprising at least oneadditional power exchange unit for forming a first set of n−1 powerexchange units, the switches of the n−1 power exchange units being ableto connect and disconnect respectively n−1 conductors of a phase linebelonging to the segment, said n−1 conductors being short-circuitedamong each other on a side of the switches.
 10. Apparatus according toclaim 9, comprising at least one additional power exchange unit forforming a second set of n−1 power exchange units, the switches of then−1 power exchange units of the second set being able to connect anddisconnect respectively n−1 conductors of a second phase line belongingto another segment, said n−1 conductors of the second phase line beingshort-circuited among each other on a side of the correspondingswitches, the first and second sets of power exchange units sharingcommon electric components for allowing a power flow between the twosegments.
 11. Apparatus according to claim 1, wherein the electriccomponent is selected among the following components: a capacitor, abattery, an inductance, a resistance, and a resistance connected inparallel to a capacitor.
 12. Method for modifying a power flow in asegment of an electric power line, the segment having phase lines eachhaving n conductors, where n is equal to or greater than 2, insulatedfrom one another and short-circuited at ends of the segment, the methodcomprising the following steps: a) providing a power unit having a powerconverter for converting power between first and second pairs ofterminals, and an electric component connected to the second pair ofterminals and capable of circulating power through the power converter,the first pair of terminals being connected in series with at least oneconductor of the n conductors of one of the phase lines; and b)controlling the power converter for modifying said power flow. 13.Method according to claim 12, further comprising step c) of selectivelyconnecting and disconnecting the first pair of terminals in series withsaid at least one conductor of the segment, by means of a switch inresponse to control signals, said switch being part of the powerexchange unit.
 14. Method according to claim 13, wherein: in step a),n−1 power exchange units are provided; in step b), the power isconverted by at least one of the n−1 power converters; and in step c),the n−1 power exchange units are connected and disconnected by means oftheir switch respectively to n−1 conductors of a phase line belonging tothe segment.
 15. Method according to claim 13, wherein: in step a), atleast one additional power exchange unit is provided for forming a setof n power exchange units; in step b), the power is converted by atleast one of the n power converters; and in step c), the n powerexchange units are connected and disconnected by means of their switchrespectively to n conductors of a phase line belonging to the segment.16. Method according to claim 13, wherein: in step a), an additionalpower exchange unit is provided for forming a set of two power exchangeunits; in step b), the power is converted by means of two powerconverters; and in step c), the two power exchange units are connectedand disconnected by means of their switch respectively to conductors oftwo phase lines belonging respectively to said segment and to anothersegment, the two power exchange units sharing a common electriccomponent for allowing a power flow between the two segments.
 17. Methodaccording to claim 16, wherein in step c), the switch of each powerexchange unit connects and disconnects, for the corresponding phaseline, n−1 conductors that are short-circuited among each other on eachside of the switch.
 18. Method according to claim 16, wherein in stepc), the switch of each power exchange unit connects and disconnects, forthe corresponding phase line, more than one and at most n−1 conductorsthat are short-circuited among each other on each side of the switch.19. Method according to claim 13, wherein in step c), the switchconnects and disconnects more than one and at most n−1 conductors of aphase line belonging to the segment, said more than one and at most-1conductors being short-circuited among each other on each side of theswitch.
 20. Method according to claim 13, wherein: in step a), at leastone additional power exchange unit is provided for forming a first setof n−1 power exchange units; in step b), the power is converted by atleast one of the n−1 power converters; and in step c), the n−1 powerexchange units are connected and disconnected by means of their switchrespectively to n−1 conductors of a phase line belonging to the segment,said n−1 conductors being short-circuited among each other on a side ofthe switches.
 21. Method according to claim 20, wherein: in step a), atleast one additional power exchange unit is provided for forming asecond set of n−1 power exchange units; in step b), the power isconverted by at least two of the power converters that belongrespectively to the first and second sets and that are linked by acommon electric component; and in step c), the n−1 power exchange unitsof the second set are connected and disconnected by means of theirswitch respectively to n−1 conductors of a phase line belonging to asecond segment, said n−1 conductors of the phase line belonging to thesecond segment being short-circuited among each other on a side of thecorresponding switches, said at least two power converters allowing apower flow between the two segments.